There are no signs, usually someone who's into BDSM is not as open about it, you know since it's such a private side of you.
There's nno handbook that you can read, just find someone you like, if the relationships blossoms then you as you earn her trust can bring up BDSM, talk about it, give her books to read but no 50 shades, I personally recommend The Masters of the Shadowlands by Cherise Sinclair. Remember also that no book about BDSM will have ALL the information there is about it.
Try to build trust, since it's one of he major things in a D/s relationship or in any relationship for that matter. If you encounter someone who after bringing up BDSM and try a few things with her and still she's not into it, well you know that a D/s relationship won't happen.
No one will start as a slave right off the bat, at the beginning the submissive is in instruction which is a period of time in which the dominant teaches her about not only what he likes, but what she likes, test her limits, learns also what is the best way to punish her, etc. After the instruction period is over the submissive asks the dominant to serve him (obviously still as a submissive) after a period of time which varies between subs, she if sees it fit, can become a slave. A slave is a sub who doesn't have any limit or safe word.